Rood et al. U.S. Pat. No. 6,364,936, incorporated by reference herein, provides for selective sorption and desorption of gases with an electrically heated ACFC element. In the '936 patent, an adsorption/desorption unit includes a hollow enclosure containing one or more elongated hollow elements of ACFC of appropriate length to cross sectional area to provide suitable electrical resistance for heating. The elements conduct electrical current to heat to a temperature that permits selective adsorption of a gas stream constituent and subsequent desorption to recover a sorbate. An enclosure houses the ACFC elements and is arranged to direct gas stream flow through the elements and into and out of the enclosure via gas ports. The ability to heat the elements to a desired temperature by electrical current flow allows for straightforward implementation of selective adsorption. After an adsorption cycle, altering the temperature of the element or elements enables desorption. A thermocouple is used to monitor temperature of the ACFC in the enclosure. In a particularly preferred embodiment, the enclosure with the element also includes a liquid condensate outlet and liquid sorbate is directly recovered as liquid from the same unit used for adsorption and desorption.
Rood et al. U.S. Pat. No. 8,080,095, incorporated by reference herein, provides a steady state tracking desorption system and method. In the '725 application, a steady state tracking desorption system achieves steady tracking of either a fixed sorbate output set point, or a set point that changes over time. The system includes an electrically heated thermal adsorption/desorption device. A temperature sensor, such as a thermocouple, senses the temperature of an adsorbent material within the adsorption/desorption device. A sorbate sensor, such as a hydrocarbon sensor, senses a sorbate concentration from an outlet of the adsorption/desorption device. A power sensor senses the power supplied by the desorption device. A controller interprets levels sensed by the temperature sensor, the sorbate sensor and the power sensor and provides a signal to achieve steady set point tracking of a sorbate concentration from the outlet of the adsorption/desorption device.
Rood et al. U.S. Patent Application 20110132031, incorporated by reference herein, now U.S. Pat. No. 8,500,853, provides gas purification methods and systems for the recovery and liquefaction of low boiling point inorganic and organic gases, such as CO2, NH3, chlorofluorocarbons, methane, and propane. Low boiling point gases are adsorbed with activated carbon fiber material during an adsorption cycle. During a desorption cycle, the activated carbon fiber is heated to a temperature to regenerate the fibers and generate a gas stream enriched with the low boiling point gases. This desorption gas stream is actively compressed and/or cooled to condense and liquefy the low boiling point gases. These methods and systems can provide recovery and liquefaction of low boiling point inorganic and organic gases, such as CO2, NH3, chlorofluorocarbons, methane, and propane. The low boiling point gases can be collected, stored, re-used, sold, etc.
Optimizing performance of ACFC thermal swing systems like those discussed above requires accurate monitoring of the temperature of ACFC cartridges during adsorption and desorption cycles. This is accomplished in the above patents with a temperature sensor, such as a thermocouple, that directly contacts the ACFC cloth.
The parent of the present application, directed toward the recovery of low boiling point gases, also uses thermocouples for temperature sensing. However, the parent of the present application also discloses using resistance measurement to determine the temperature during the desorption cycle when electrical power is being applied to heat the ACFC. This is disclosed, for example, in [0023] of US Patent Application 20110132031. The resistance based sensors functions well and reduce the need for direct temperature sensors, e.g., thermocouples, that are attached to the adsorbent. As discussed in [0094] of the '031 published application, thermocouples can provide incorrect values, and fail due to aging or required maintenance operations. Such thermocouples can be damaged during resistive heating for regeneration.
Commercially used systems typically use beds that are not electrically heated, which provides less control than the above Rood et al. devices, and also have additional drawbacks compared to the above discussed systems often require downstream sensing to determine when regeneration or replacement of adsorption material is necessary, or when destruction equipment must be adjusted. All known commercial systems known to the inventors that use different types of adsorbents and adsorbent heating processes also use downstream hydrocarbon sensors.
Various systems are commercially in use in a wide variety of manufacturing industries to reduce emissions of volatile organic compounds (VOCs). Environmental control devices, such as thermal oxidizers or vessels containing granular activated carbon (GAC) are examples. Thermal oxidizers require costly auxiliary fuel and convert VOCs to H2O, CO2, and NOX. GAC allows for capture and recovery of VOCs, but GACs are known to ignite during adsorption cycles when treating a wide range of VOCs such as ketones. Non-regenerable GACs also require additional cost for replacement and disposal of the saturated adsorbent, which is often categorized as a hazardous waste. Capture and recovery of these VOCs for reuse without costly ignition issues reduces atmospheric emissions and improves air quality while providing feedstock for reuse reducing manufacturing cost and conserving materials.
Various sensors in the above discussed oxidizer, ACFC and GAC systems can be expensive and require frequent maintenance. Direct temperature (i.e. thermocouples) sensors used in ACFC systems can fail. For example, thermocouples in contact with an adsorbent have been shown to periodically fail from loss of adsorbent contact and from shorting the electrothermal regeneration circuit resulting in burning of the thermocouples and ACFC, which can necessitate repairs and reduced system operating time.
Hydrocarbon sensors used with adsorption and thermal oxidizer systems can also fail. The hydrocarbon sensors used in the adsorption and thermal oxidizer systems are expensive (>$10K), require weekly calibration, and periodically fail. For example, infrared hydrocarbon sensors that operate with GAC control devices used for aircraft coating operations at Hill Air Force Base (HAFB), Utah typically fail after 2-4 months of operation.
The general cycles in the various systems include adsorption, regeneration, and cooling cycles. Adsorption and regeneration cycles are typically controlled based on measurements from hydrocarbon sensors, which requires their initial purchase, maintenance, and periodic calibration. Other systems also use local temperature sensors (e.g., thermocouples) to control electrothermal heating during regeneration cycles and to determine when a cooling cycle is complete.
Patents and publications concerning bed-based GAC have used direct contact local resistance to measure loading. For example, Puskas, U.S. Pat. No. 6,593,747 discloses a system that uses a packed bed of carbon particles. Local conductivity measurements are taken with pairs of opposing electrodes that are immersed in the packed bed. The local conductivity measurements are correlated to local saturation of the packed bed. The same technique is also disclosed by Puskas, Del Vecchio, et al., “New Method for Monitoring of Adsorption Column Saturation and Regeneration I. Demonstration of the Measurement Principle,” Chemical Engineering Science 59 2389-2400 (2004). Similarly, N.D. Del Vecchio, Puskas and Barghi, “New Method for Monitoring of Adsorption Column Saturation and Regeneration I. Demonstration of the Measurement Principle,” Chem. Eng. Comm., Vol. 189(3), pp. 352-71 (2002), discloses measurement of the resistance of the local area in a packed bed of adsorbent particles between two parallel plates that contact the particles.
Selective adsorbent activity characterization in the prior work discussed in the previous paragraph shows a change in electrical resistance measured locally with direct contact to a packed adsorbent bed that occurs during adsorption, which can be used to determine the end of an adsorption cycle and when regeneration of the adsorbent is complete. This process, in addition to requiring direct electrical contact with a packed bed, fails to account for dependence of resistance on temperature. It is therefore limited to systems that operate at constant temperature and cannot be utilized for commonly used thermal swing adsorption systems (e.g., electrothermal swing adsorption (ESA) system), which are typically more cost efficient for treating gas streams with high flow rates (>few thousand m3/hr). This relationship between electrical resistance and adsorbed mass at constant temperature has also been previously described by others. See, R. Haines, R. S. Benson, G. C, “The effect of physical adsorption on the electrical resistance of activated carbon” Journal of Chemical Physics, 15, (1), 17-27 (1946).
ACFC is known to act as a typical semiconductor at relevant adsorbent temperatures (20-200° C.), such that resistance decreases as temperature increases. Electrical current and voltage measurements are typically required and used for electrothermal heating in known devices.